I wouldn't be too worried or concerned about that. (Especially when I own and enjoy both systems.)
I expect the PS4 to be the winner in the end. I've got it pegged for 125 million, though that may also change depending on 2018.
For the most part, the PS4 and Switch are targeting two different audiences and demographics, this is evidenced by how the PS4 is enjoying its peak year by far, while the Switch has far surpassed even Nintendo's original expectations and is off to one of the best starts we've seen from a video game system.
If anything, I expect crossover appeal between the two, because both have amassed a strong lineup of 1st party titles that would entice people to buy their systems for the first party content alone. I could see people who own a PS4 as their main console will want to invest in a Nintendo Switch down the line for two reasons: The handheld component it shares with its handheld predecessors and so they can have access to HD console level/quality Mario, Zelda, and Pokemon games. And I can see people who own a Switch wanting to invest in a PS4 so they can have access to Sony's diverse and ever-growing 1st party lineup.
I'm sure that these systems are taking sales away from one another, as would be expected from competition. But, I think that it is minimal and not significant enough to pay attention to. There's more than enough room for the PS4 and Switch to succeed simultaneously, and for Nintendo and Sony expand and grow in the market... At the expense of Microsoft.